1. Field of the Invention
The present invention relates to a magnesia granule and a process for the production thereof. The magnesia granule of the present invention is useful as a filler of a resin composition for sealing electronic parts or the like, has an excellent thermal conductivity, resistance to deterioration, and insulating properties, and does not emit o rays, which may cause operation errors in semiconductor elements.
2. Description of the Related Art
Hitherto, crystalline silica and fused silica have been used as a filler of a resin composition for sealing electronic parts, but since electronic parts are now highly integrated with semiconductor elements and the like, thus drastically increasing the quantity of generated heat, silica can not sufficiently dissipate this heat due to a relatively low thermal conductivity thereof. Furthermore, silica has a drawback in that, since the hardness thereof is high, a resin-sealing material containing silica causes serious wear of a molding machine and mold during resin-sealing by transfer molding or injection molding.
As a filler material capable of overcoming these drawbacks, magnesia is receiving great attention due to the high thermal conductivity and low hardness thereof, but magnesia has the following drawbacks, which impede the practical use thereof.
(1) A magnesia powder prepared by pyrolytically decomposing a magnesium salt synthesized by using a magnesium ion in seawater as a starting material, and then powdering the decomposed material, contains uranium and thorium in an amount as high as several hundred ppb. Therefore, the magnesia powder cannot be used as desired because the sealed storage elements cause operation errors due to .alpha. rays emitted from the uranium and thorium. Furthermore, the magnesia powder prepared by pyrolytic decomposition has a great tendency to secondarily agglomerate and exhibits an unsatisfactory dispersion when incorporated into a resin.
(2) A magnesium powder prepared by gas phase oxidation of a heated vapor of metallic magnesium can have a reduced content of uranium and thorium of as low as about 1 ppb, and has an excellent dispersibility, but since the powder is prepared by a build-up process, it is difficult to obtain coarse particles, and thus a powder of particles having an average particle size of several micrometers or more cannot be prepared. Therefore, since the powder has a fine particle size, it is difficult to handle and to satisfactorily effect the incorporation thereof with a resin.
(3) If a magnesia powder prepared by the gas phase oxidation is granulated into a particle size of several tens of micrometers by a conventional method such as spray drying, the handling properties can be improved, but the granular particles are broken during the incorporation with a resin and become dispersed in the resin as the primary particles of the starting powder, to strikingly increase the viscosity, and thus the incorporating operation becomes very difficult or even impossible.
(4) Magnesia is relatively easily hydrated by moisture in air and converted into magnesium hydroxide, and therefore, when incorporated with a resin, the conversion into magnesium hydroxide is caused by moisture permeating into the resin from the air, to cause cracks in the resin due to a volume expansion of the magnesia due to the conversion into magnesium hydroxide, thereby degrading the long term reliability of the sealed electronic parts.